Multilevel inverters are widely used in high-power high-voltage applications due to advantageous performance over two-level inverters, including reduced voltage pressure or tension on the power devices, lower harmonics, lower instantaneous rate of voltage change (dv/dt), and lower common-mode voltage.
Among various modulation strategies for multilevel inverters, space vector pulse width modulation (SVPWM), provides significant flexibility to optimize switching waveforms, and is suitable for implementation in digital signal processors. For an n-level inverter, there are n3 switching states and 6(n−1)2 modulation triangles in the space vector diagram. A reference vector defining desired switching state of the inverter can be placed at any modulation triangle of the space vector diagram. To reduce the harmonics and voltage surges during the switching transients, the Nearest Three Vectors (NTV) is commonly adopted. According to the NTV approach, the reference vector of the voltage is equivalent to the nearest three vectors in terms of the average voltage during a switching cycle. However as the level of the inverter increases the increased number of triangles, switching states, and calculation of duty cycles enlarges the complexity of SVPWM for multilevel inverters.
There are two common methods of SVPWM for multilevel inverters. The first method determines the modulation triangle, and then solve three simultaneous equations for that triangle to obtain the duty cycles, see T. Ishida, et al., “A control strategy for a five-level double converter with adjustable dc link voltage,” Proc. Ind. Appl. Conf., October 2002, vol. 1, pp. 530-536. The second method determines the modulation triangle, and then uses the particular duty cycle equations pre-stored in the lookup table for this triangle, see S. Mondal, et al., “A neural-network based space-vector PWM controller for a three-level voltage-fed inverter induction motor drive,” IEEE Trans. Power Electron., vol. 38, no. 3, pp. 660-669, May/June 2002. However, with the increasing number of levels of the inverter, both of those two methods become intensive in computation.
Several SVPWM based methods are known for three-level inverters. However, those methods are not readily extended to four or higher level inverters. For example, one method partitions the three-level space vector diagram into six two-level space vector diagrams, see H. Zhang, et al., “Multilevel inverter modulation schemes to eliminate common-mode voltages,” IEEE Trans. Ind. Appl., vol. 36, no. 6, pp. 1645-1653, November/December 2000. In H. Zhang's method, the axes of the d-q plane are rotated by a certain angle in each calculation of the reference vector location, and no general method for switching sequence selection or application for four or higher level inverter is introduced.
A similar method for three-level inverter is described by J. Seo, et al., “A new simplified space-vector PWM method for three-level inverters,” IEEE Trans. Power Electron., vol. 16, no. 4, pp. 545-550, July 2001. In J. Seo's method, a two-phase to three-phase conversion is performed to calculate the shift of origin of a virtual two-level inverter. After the shift of origin and 60° coordinate transformation, duty cycles are calculated using two-level equations. Because of the two-phase to three-phase conversion for each partition of the space vector diagram, the complexity and computation of the method are increased when applied to a four or higher level inverter. Moreover, no general switching sequence selection is used by the method.
A Euclidean vector system based SVPWM is describe by N. Celanovic, et al., “A fast space vector modulation algorithm for multilevel three phase converters,” WEE Trans. Ind. Appl., vol. 37, no. 2, pp. 637-641, March/April 2001. However, several matrix transformations are needed, and no systematic approach for determining the switching states or real-time implementation is provided in N. Celanovic's method. A coordinate transformation and switching sequence mapping based SVPWM scheme is described by A. Gupta, et al., “A space vector PWM scheme for multilevel inverters based on two-level space vector PWM,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1631-1639, October 2006. In A. Gupta's method, a coordinate transformation is needed to determine the location of the reference vector and to calculate the duty cycles, and a pre-stored switching sequence mapping table is needed to determine the switching sequence. However, because the number of possible switching sequences increases with the increasing level of the inverter, more memory is needed and slower mapping speed is achieved when A. Gupta's method is applied to higher level inverters.
Accordingly, there is a need for general SVPWM based method for multilevel inverters.